Input.get_accelerometer() and Input.get_gravity() need further description

[celyk]

Your Godot version:
v4.2.1.stable.official [b09f793f5]

Issue description:
I recently learned that these acceleration vectors have two conventions depending on platform.

Android: This is the proper acceleration of the device in units of m/s².
iOS: Like that but negated and in units of g instead. This is the acceleration of free falling objects relative to the device.

Godot is a mix of both. It uses the negated acceleration in m/s². The docs make no mention of the negation.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_input.html#class-input-method-get-accelerometer
https://docs.godotengine.org/en/stable/classes/class_input.html#class-input-method-get-gravity